"Since...November 2010, the most common Flash Player zero-day attack vector has been malicious Flash content embedded in Microsoft Office documents and delivered via email," said Peleus Uhley, a platform security strategist, in a Feb. 7 post to the Adobe Secure Software Engineering Team (ASSET) blog.

Uhley also noted that the latest round of zero-day vulnerabilities in Flash -- ones criminals are already using by the time bugs are patched -- were being exploited with that tactic.

To make Office document-based attacks more difficult, documents generated with Office 2007, 2003 and XP -- Microsoft dropped the latter from its support list in mid-2011 -- will no longer auto-execute Flash content. Instead, when documents created with those suites are opened, Flash will display a dialog recommending the user not let the content play.

Users can select a second option and play the Flash content if they're certain the document came from a trusted source.

Flash won't bother showing the pop-up for documents generated with newer versions of Office, including Office 2010 and the just-released Office 2013, because those suites sport a sandbox that prevents automatic play of Flash content.

Microsoft calls its Office sandbox "Protected View," a feature introduced in Office 2010 and continued in Office 2013. In both suites, Word, Excel, and PowerPoint files are opened in a sandboxed, or isolated, instance of the application when the file was downloaded from the Internet or opened as an Outlook email attachment.

Microsoft did not back-port the Protected View sandbox to older editions of Office.

Adobe's Uhley did not promise that the new dialog would stop Flash exploits embedded in Office documents, but was instead simply crossing his fingers. "We've seen these types of user interface changes lead to shifts in attacker behavior in the past, and are hopeful this new capability will be successful in better protecting Flash Player users from attackers leveraging this particular attack vector as well," he said.

Utley did not provide a timetable for when the security pop-up feature would hit Flash Player, saying only that it would appear in the next "feature release" of the software, "which is currently in beta."

Adobe will try to block some attacks exploiting Flash in Office documents with this click-before-play warning. The feature will debut in Flash Player 11.6, which is now in beta testing. (Image: Adobe.)

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